CHAP. IV.] EVOLUTION THE KEY TO DISTRIBUTION. 



61 



that species vary in these more important as well as in the 

 more superficial characters. If, then, in any part of the area 

 occupied by a species some change of habits becomes useful 

 to it, all such structural variations as facilitate the change will 

 be accumulated by natural selection, and when they have be- 

 come fixed in the proportions most beneficial to the animal, we 

 shall have the first species of a new genus. 



A creature which has been thus modified in important 

 characters will be a new type, specially adapted to fill its 

 place in the economy of nature. It will almost certainly 

 have arisen from an extensive or dominant group, because 

 only such are sufficiently rich in individuals to afford an ample 

 supply of the necessary variations, and it will inherit the 

 vigour of constitution and adaptability to a wide range of 

 conditions which gave success to its ancestors. It will there- 

 fore have every chance in its favour in the struggle for existence ; 

 it may spread widely and displace many of its nearest allies, 

 and in doing so will itself become modified superficially and 

 become the parent of a number of subordinate species. It 

 will now have become a dominant genus, occupying an entire 

 continent, or perhaps even two or more continents, spreading in 

 every direction till it comes in contact with competing forms 

 better adapted to the different environments. Such a genus 

 may continue to exist during long geological epochs ; but the 

 time will generally come when either physical changes, or 

 competing forms, or new enemies are too much for it, and it 

 begins to lose its supremacy. First one then another of its 

 component species will dwindle away and become extinct, till 

 at last only a few species remain. Sometimes these soon follow 

 the others and the whole genus dies out, as thousands of genera 

 have died out during the long course of the earth's life-history ; 

 but it will also sometimes happen that a few species will 

 continue to maintain themselves in areas where they are removed 

 from the influences that exterminated their fellows. 



Cause of the Extinction of Species. — There is good reason to 

 believe that the most effective agent in the extinction of species 

 is the pressure of other species, whether as enemies or merely 

 as competitors. If therefore any portion of the earth is cut off 



